1. Field of the Invention
The present invention relates to a method of growing silicon single crystals by the Czochralski method and more particularly, to the method of growing single silicon crystals, where the shape of a portion from a seed crystal to the lower end of a neck is specialized so as to increase the mechanical strength of the neck and thereby to make it possible to grow single silicon crystals of a larger diameter and heavier weight.
2. Description of the Prior Art
There is well known a method of growing a dislocation-free silicon single crystal by the Czochralski method, where single crystal silicon is used as a seed crystal and the seed crystal is contacted to a silicon melt, pulled up to grow a silicon single crystal at the lower end, the single crystal is squeezed in diameter to form a neck and thus to eliminate dislocations therein generated when the seed crystal contacts the melt, and thereafter the dislocation-free single crystal is continuously grown to get larger in diameter.
In this method, a near-cylindrical portion called a neck is preferably so formed as to have the smallest diameter practically possible in order to achieve the condition of being dislocation-free therein by necking or squeezing the diameter of the silicon single crystal following the seed crystal. For example, growing of the neck is performed with the dimensions and shape of less than about 3 mm in diameter and of about 30 mm to 200 mm in length.
Recently, silicon single crystals of a larger diameter have become increasingly required as starting material for semiconductor devices in order to realize high throughput and yield together with ever complicating designs in the device chips, which trend automatically entails requirements for a heavier weight of a silicon single crystal.
In this environment, the mechanical strength of a neck is more demanding, since all the weight of a growing crystal have to be supported during growth by the neck, which means a portion from the lower portion of a seed crystal to the lower end of the neck.
A simple solution to have a higher mechanical strength of the neck is to make the diameter of the neck larger throughout all the length thereof. Simply making the diameter larger has, however, inevitably a resulting difficulty in growing a larger diameter single crystal free of dislocations since the neck could not be grown free of dislocations and thereby the mechanical strength thereof is lowered, even though the mechanical strength of a squeezing portion itself may be increased, which is a tapered portion in transition from starting of a squeeze of the seed diameter to the upper end of the near-cylindrical neck, (hereafter referred to as a seed taper).
In the above-mentioned traditional way, the diameter of a neck is controlled to be less than about 3 mm so as to meet the condition for being free of dislocations and consequently the neck does not provide a strength enough to raise a heavy growing single crystal. With this size of the neck diameter, there is a limit of the diameter of a grown single crystal, say, at most of about 150 mm or 6 inches for usual lengths of grown single crystals and growing a single crystal of more than about 200 mm or 8 inches in diameter automatically forces to shorten the length available thereof due to indispensable total-weight limitation thereof, which results in poorer throughput and yield. Growing a single crystal of a larger diameter in excess of the limit causes a stress within the bulk of the neck beyond endurance, which naturally means a strong possibility of being severed in the neck in the course of growth and as a result leads to a terrible accident of free falling of a single crystal.
In addition to the above problem, the inventors have discovered in the course of the investigation, which finally brought them to make the present invention, another technical problem that a neck free of dislocations is week to plastic deformation and slippage both due to a tensile stress caused by a heavy weight of a single crystal, and due to higher temperatures of the surrounding working environment. The neck affected by slippages is deteriorated in mechanical strength and thus the probability to run a risk of free falling of a grown crystal due to severance in the neck is adversely increased.
What's more, in conventional methods of this kind, a variety of choices are adopted in regard to the shapes of seed taper, the length and diameter of a neck and there is, as a result, an inability in exactly locating where the condition of being dislocation-free starts in necking during growth. This additional week point of the conventional methods makes themselves unable to find out a way to delete the mechanically week portion from a necking process.
Specially designed supporting devices of a silicon single crystal might have a chance for the use in solving the disadvantages above-mentioned. The devices of these kinds are, however, not available with practicality in terms of cost and efficiency and have a plurality of technical problems, for example, of excessive cost, complexity of operation, impurities' contamination or the like in use.